Detection of subsurface defects in machined silicon nitride ceramics by optical scattering methods

摘要

Si(sub 3)N(sub 4) materials, from two manufacturers in the form of texture test bars, were studied by optical Fourier scattering methods. Subsurface defects were synthesized by either ultrasonically drilling or by grinding slots parallel to the surface. Because of unique optical transmission properties of ceramics subsurface defects, such as those likely to be caused by machining may be detected. S-polarized He-Ne laser light ((lambda) = 0.6328 (mu)m) was used as the light source. Specular reflections were detected by an optical Fourier method which incorporated cross-polarization. Analysis of scattered laser light was accomplished using a CCD-array camera, an 8-bit A/D converter, and a PC with locally written software. Results show that subsurface defects can be detected and that for the surface finishes studied, changes in signal-to-noise (S/N) are observable.